Wheels are normally manufactured from two components: a central disk or "spider" which is press fitted and welded inside a tire-supporting rim. The rim and disk each possess a number of significant surface features which, in order to produce a satisfactory wheel, must be properly shaped and/or maintained in a desired positional relationship with other surface features of the same component or, in the case of an assembled wheel, positioned correctly with respect to certain surface features of its mating component.
For instance, the disk usually contains a central pilot hole adapted to receive the center flange of a vehicle hub. The pilot is usually surrounded by a so-called "bolt circle" which comprises an array of four or five mutually spaced smaller holes ringing the pilot hole. The holes in the bolt circle receive lug bolts for effectively securing the disk to the hub. The bolt circle must be maintained concentric with the pilot hole within controlled tolerances.
The rim component of a wheel also includes a number of surface features of significance including pairs of opposed bead seats and safety humps, respectively, each of which must be of a proper diameter and properly centered with respect to one another on the rim. When the wheel is assembled, it is important that bead seats on the rim also be positioned properly with respect to the pilot and/or bolt circle on the disk. It is ordinarily desired that the bead seat be either concentric with the pilot or maintained in some specified non-concentric relationship with the pilot. Therefore, it is necessary to measure wheel components in order to ensure compliance with specifications. A wheel manufacturer may use measurement data so obtained not only to reject unacceptable wheels but also to look for trends in tolerances in order to maintain control production using modern statistical process control (SPC) techniques.
A known machine for measuring wheels included a rotatable spindle adapted for mounting either an expandable collet centeringly engageable with the pilot or bolt circle fixturing centeringly engageable with the bolt circle holes. With the collet mounted on the spindle, the wheel was mounted upon the collet, and the collet expanded into engagement with the edge of the pilot to center the pilot on the spindle. By rotating the wheel with a distance measuring probe engaging its bead seat, the distance between points along the bead seat and the center of the pilot were then measured and the measurement data stored in a computer connected to the probe. The rotation of the wheel was then stopped, the wheel was demounted from the collet and the collet then demounted from the spindle. Bolt circle fixturing was then mounted on the spindle in place of the collet and the wheel then mounted on the bolt circle fixturing to center the bolt circle on the spindle. The distance between the points on the bead seat and the center of the bolt circle were then measured again. The runout between the bolt circle and the pilot was then calculated by the computer by algebraically combining vectors representing the first harmonics of the two sets of measurement data.
The need to stop rotation of the spindle, demount the wheel, exchange fixtures, remount the wheel on the new fixture and resume rotation of the spindle prior to commencing measurements with reference to the center of a different surface feature render the operation of such machines inefficient and therefore costly.
It has also been known to provide a machine having a rotatable spindle equipped with pilot tooling to center the pilot on the axis of rotation of the spindle. With the pilot so centered on the spindle, a separate bolt circle measuring fixture was mounted in engagement with the bolt circle. The measuring fixture took the form of a precisely circular disk having fingers extending perpendicularly from a face of the disk to center the disk on the bolt circle. A distance measuring probe engageable with the peripheral edge of the disk could be used to measure the runout of the bolt circle with respect to the pilot directly. Similarly, a second distance measuring probe engageable with the bead seat was provided for direct measurement of the runout between the pilot and the bead seat.
While eliminating the need to demount and remount the wheel and exchange fixturing during a measuring operation, the latter machine suffered from a significant drawback in that it proved difficult to consistently mount and maintain the measuring surface of the bolt circle fixture perpendicular to the spindle axis of the machine. As a result, the distance measurements made with the probe would often include an erroneous wobble component.